Development of a dynamic cumulative damage model and its application to underground hydropower caverns under multiple blasting

Underground infrastructures are crucial for resource extraction, energy storage, and space utilisation. The geomaterials that make up these structures, such as rock and concrete, are subjected to multiaxial stress conditions and are frequently exposed to dynamic and extreme loadings caused by both natural disasters and human activities. These stresses are particularly significant during the construction phase, which involves operations such as drilling and blasting excavation, as well as during the operational phase, which may include events like explosions. For instance, while drilling and blasting induce rock breakage within the excavated profile as designed, they inevitably lead to the formation of a damage zone in the surrounding rock mass. Moreover, the cumulative effects of sequential excavations and multiple blasts can cause significantly greater damage, thereby threatening the stability of tunnel structures during the operation phase. This paper highlights the importance of thoroughly analysing these phenomena during both static and dynamic loadings to ensure the stability of underground infrastructures. To address these challenges, a rate-dependent damage constitutive model is proposed for geomaterials to assess the impacts of blasting loads and the cumulative damage resulting from repeated blasts. The model is conceptualised using the strength envelope of loading-unloading curves to represent the progressive accumulation of damage under repeated impacts. Through theoretical derivation, a dynamic cumulative damage model is developed, based on a modified Mohr-Coulomb strain-softening model incorporating rate-dependent parameters, and is validated against dynamic experimental data. The model captures the transition between static strain-softening and dynamic cumulative damage, triggered by a critical strain-rate threshold. The applicability of the model is demonstrated through simulations of tunnel excavation, emphasising the impact of blasting loads and the accumulation of damage zones. To assess its practical feasibility, the developed model is applied to simulate different excavation scenarios for an underground hydropower cavern. Damage in the surrounding rock mainly results from static unloading and/or dynamic disturbances. Blasting construction, in particular, causes significant damage in tunnel intersection zones and the connecting areas of two benches, leading to increased displacement and higher damage levels compared to static excavation. To mitigate excessive damage while maintaining the construction timeframe, it is recommended to consider alternating cycles of dynamic loading and static excavation unloading continuously, which helps understand damage formation in critical zones without significantly delaying project completion.